Cluster dynamics simulation of Zr hydrides formation on grain boundaries in Zr

Abstract

The delta hydrides formation in zirconium cladding under irradiation and thermal aging is studied by the rate theory modeling. The precipitates may lead to cracking of the clad and are investigated both experimentally and theoretically. The rate theory studies published so far are based on the well-established classical nucleation theory, but use several assumptions, e.g. of homogeneous hydride nucleation mode and spherical shape of precipitates, which contradict observations. In this work we continue the development of the hydride precipitation model by taking into consideration the observed preferential nucleation of the platelet-shaped hydrides on grain boundaries, and analyze the importance of such modifications by comparing calculations performed with different approaches. We also extract information from the literature on the hydrogen solution energy and alpha Zr / delta hydride interface energy, which define the hydrogen-hydride binding energy as a function of precipitate size, and show that the observed hydride size and density are well described with the use of the capillary approximation. The difference in precipitation kinetics during thermal annealing of samples with pre-existing level of hydrogen and under irradiation to the same final hydrogen concentration is elucidated. This improves our understanding of the factors affecting hydride formation in the clad.